Quantum Chemistry of CO2 Interaction with Swelling Clays
Ubiquitous clay minerals can play an important role in assessing the suitability of geologic formations for secure storage of carbon dioxide (CO2). The minerals may affect the reservoir storage capacity as well as the integrity of its natural seals such as caprock formations. CO2 interaction with swelling clays such as smectites is a complex process involving physisorption in micropores and intercalation (insertion) of CO2 molecules between the layers of the clay. In montmorillonite—a swelling clay that is a member of the smectite group of clays—the width of the interlayer gap is controlled by polar water molecules hydrating the cations between layers. As the gap becomes wider the energy required for CO2 intercalation decreases.
|Dioctahedral smectites have a lamellar structure with an octahedral sheet of repeating AlO6 units between two tetrahedral SiO4 sheets. Isomorphic metal-ion substitutions (e.g., Al3+replaced by Mg2+) result in fixed-charge imbalance that is compensated by hydrated metal cations (e.g., Ca++) residing between the negatively charged clay layers.
NETL researchers were pioneers in developing basic science involving high-pressure CO2 interaction with montmorillonite. To explain the findings of analytical techniques showing the CO2 sorption isotherm hysteresis, changes in basal distances between the clay layers detected by X-ray diffraction, and unusual infrared “fingerprints” attributed to CO2 molecular vibrations in the clay interlayer, NETL Geosciences researcher Slava Romanov and Geological and Environmental Sciences Focus Area Lead George Guthrie promoted broad collaboration across several DOE national laboratories and the NETL–Regional University Alliance (RUA).
A collaborative effort between NETL–RUA and Sandia National Laboratories resulted in a complete and consistent model of CO2 intercalation and subsequent carbonation steps within montmorillonite interlayers supported by several complementary experimental techniques and molecular dynamics simulations. Additionally, proof-of-concept experimental and theoretical work at PNNL contributed to identifying several distinct CO2 intercalation mechanisms. PNNL researchers confirmed NETL reports of bi-modal asymmetric stretch vibrations observed in the infrared spectra of the trapped CO2 and discovered that the observed vibrations are associated with in-plane and perpendicular orientations of intercalated molecules that had been previously hypothesized by NETL researchers. Such multi-laboratory collaborations accelerate advances in research. The interaction of CO2 with critical components of shale formations, such as clays, plays a significant role in carbon storage and utilization.
Contact: Slava Romanov, 412-386-5476